Several systems use ultrasonic sound waves to measure the velocity of a fluid through a conduit. In Takada, U.S. Pat. No. 4,337,667, an ultrasonic flowmeter is disclosed which uses a phase difference method to calculate the velocity of a fluid through a conduit. The ultrasound flow rate meter includes a single transmitter for transmitting a continuous ultrasonic wave into a fluid flowing through a conduit and a single receiver spaced a predetermined distance from the transmitter for receiving the ultrasonic wave. The phase difference between the signals at the transmission point and the reception point is measured and used to calculate the velocity of the fluid in the conduit.
One disadvantage of the prior art ultrasonic flowmeters is that compensation for temperature changes in the fluid is not provided. Compensation for temperature changes is needed because the speed at which the ultrasonic wave propagates through the fluid, and hence the phase of the signal at the reception point, is related to the temperature of the fluid. Since the phase of the signal at the reception point is used to calculate the velocity of the fluid in the conduit, temperature changes in the fluid result in erroneous changes in the measured velocity of the fluid.
Another disadvantage of the prior art ultrasonic flowmeters is that long conduits are necessary to accurately measure the fluid velocity. This is because ultrasonic sound waves reflected from the ends of the conduit become significant as the ends of the conduit are brought closer to the receiver. (The effective soundwave at the receiver is the vector sum of the soundwave transmitted from the transmitter and the reflected sound waves.)
It is desirable to have a device for measuring the air speed of an object, in particular when conventional ground speed calculations are impractical. Examples include sporting and recreational activities such as skiing and horseback riding. To accommodate such uses, it is important that the device is small, lightweight and is able to operate for an extended period of time with a minimum amount of power consumption. It is also important that the device accurately measures air speed over a wide variety of temperatures.